1. Field of the Invention
The present invention relates generally to a buoyancy system for supporting a riser of a deep-water, floating oil platform. More particularly, the present invention relates to a buoyancy system having one or more buoyancy modules including a rigid ecto-skeleton to withstand lateral or bending loads, and a buoyancy vessel to withstand internal pressure.
2. Related Art
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from the less productive oil reserves, or to reach the more distant oil reserves. Such distant oil reserves may be located below the oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves.
In addition, some oil reserves are located farther offshore, and thousands of feet below the surface of the oceans. Certain floating oil platforms, known as spars, or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend the thousands of feet to reach the ocean floor, and the oil reserves beyond.
It will be appreciated that these risers, formed of thousands of feet of steel pipe, have a substantial weight, which must be supported by buoyant elements at the top of the risers. The underlying principal of buoyancy cans is to remove a load-bearing connection between the floating vessel and the risers. Steel buoyancy cans (i.e. air cans) have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture.
In addition, as risers have become longer by going deeper, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the width of the well bays within the platform structure, while the length is limited by the practicality of handling the air cans. For example, the length of the air cans is limited by the ability or height of the crane that must lift and position the air can. Another factor limiting air can length is the distance to interference points with the platform structure below the air can. One disadvantage with more and/or larger air cans is that the additional length and larger diameter air cans adds more and more weight which also be supported by the air cans, decreasing the air can""s ability to support the risers. Another disadvantage with merely stringing a number air cans is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
Vast oil reservoirs have recently been discovered in very deep waters around the world, principally in the Gulf of Mexico, Brazil and West Africa. Water depths for these discoveries range from 1500 to nearly 10,000 ft. Conventional offshore oil production methods using a fixed truss type platform are not suitable for these water depths. These platforms become dynamically active (flexible) in these water depths. Stiffening them to avoid excessive and damaging dynamic responses to wave forces is prohibitively expensive.
Deep-water oil and gas production has thus turned to new technologies based on floating production systems. These systems come in several forms, but all of them rely on buoyancy for support and some form of a mooring system for lateral restraint against the environmental forces of wind, waves and current.
These floating production systems (FPS) sometimes are used for drilling as well as production. They are also sometimes used for storing oil for offloading to a tanker. This is most common in Brazil and West Africa, but not in Gulf of Mexico as of yet. In the Gulf of Mexico, oil and gas are exported through pipelines to shore.
Drilling, production, and export of hydrocarbons all require some form of vertical conduit through the water column between the sea floor and the FPS. These conduits are usually in the form of steel pipes called xe2x80x9crisers.xe2x80x9d Typical risers are either vertical (or nearly vertical) pipes held up at the surface by tensioning devices; flexible pipes which are supported at the top and formed in a modified catenary shape to the sea bed; or steel pipe which is also supported at the top and configured in a catenary to the sea bed (Steel Catenary Risersxe2x80x94commonly known as SCRs).
The flexible and SCR type risers may in most cases be directly attached to the floating vessel. Their catenary shapes allow them to comply with the motions of the FPS due to environmental forces. These motions can be as much as 10-20% of the water depth horizontally, and 10s of ft vertically, depending on the type of vessel, mooring and location.
Top Tensioned risers (TTRs) typically need to have higher tensions than the flexible risers, and the vertical motions of the vessel need to be isolated from the risers. TTRs have significant advantages for production over the other forms of risers, however, because they allow the wells to be drilled directly from the FPS, avoiding an expensive separate floating drilling rig. Also, wellhead control valves placed on board the FPS allow for the wells to be maintained from the FPS. Flexible and SCR type production risers require the wellhead control valves to be placed on the seabed where access and maintenance is expensive. These surface wellhead and subsurface wellhead systems are commonly referred to as xe2x80x9cDry treexe2x80x9d and xe2x80x9cWet Treexe2x80x9d types of production systems, respectively.
Drilling risers must be of the TTR type to allow for drill pipe rotation within the riser. Export risers may be of either type.
TTR tensioning systems are a technical challenge, especially in very deep water where the required top tensions can be 1000 kips or more. Some types of FPS vessels, e.g. ship shaped hulls, have extreme motions which are too large for TTRs. These types of vessels are only suitable for flexible risers. Other, low heave (vertical motion), FPS designs are suitable for TTRs. This includes Tension Leg Platforms TLP), Semi-submersibles and Spars, all of which are in service today.
Of these, only the TLP and Spar platforms use TTR production risers. Semi-submersibles use TTRs for drilling risers, but these must be disconnected in extreme weather. Production risers need to be designed to remain connected to the seabed in extreme events, typically the 100 year return period storm. Only very stable vessels are suitable for this.
Early TTR designs employed on semi-submersibles and TLPs used active hydraulic tensioners to support the risers. As tensions and stroke requirements grow, these active tensioners become prohibitively expensive. They also require large deck area, and the loads have to be carried by the FPS structure.
Spar type platforms recently used in the Gulf of Mexico use a passive means for tensioning the risers. These type platforms have a very deep draft with a central shaft, or centerwell, through which the risers pass. Buoyancy cans inside the centerwell provide the top tension for the risers. These cans are more reliable and less costly than active tensioners.
Types of spars include the Caisson Spar (cylindrical), and the xe2x80x9cTrussxe2x80x9d spar. There may be as many as 40 production risers passing through a single centerwell. The Buoyancy cans are typically cylindrical, and they are separated from each other by a rectangular grid structure referred to a riser xe2x80x9cguidesxe2x80x9d.
These guides are attached to the hull. As the hull moves the risers are deflected horizontally with the guides. However, the risers are tied to the sea floor, hence as the vessel moves the guides slide up and down relative to the risers (from the viewpoint of a person on the vessel it appears as if the risers are sliding in the guides).
A wellhead at the sea floor connects the well casing (below the sea floor) to the riser with a special Tieback Connector. The riser, typically 9-14xe2x80x3 pipe, passes from the tieback connector through the bottom of the spar and into the centerwell. Inside the centerwell the riser passes through a stem pipe, or conduit, which goes through the center of the buoyancy cans. This stem extends above the buoyancy cans themselves and supports the platform to which the riser and the surface wellhead are attached. The buoyancy cans need to provide enough buoyancy to support the required top tension in the risers, the weight of the cans and stem, and the weight of the surface wellhead.
Since the surface wellhead (xe2x80x9cdry treexe2x80x9d) move up and down relative to the vessel, flexible jumper lines connect the wellhead to a manifold which carries the product to a processing facility to separate water, oil and gas from the well stream.
Spacing between risers is determined by the size of the buoyancy cans. This is an important variable in the design of the spar vessel, since the riser spacing determines the centerwell size, which in turn contributes to the size of the entire spar structure. This issue becomes increasingly more critical as production moves to deeper water because the amount of buoyancy required increases with water depth. The challenge is to achieve the buoyancy needed while keeping the length of the cans within the confines of the centerwell, and the diameters to reasonable values.
The efficiency of the buoyancy cans is compromised by several factors:
Internal Stem
The internal stem is typically flooded and provides no buoyancy. Its size is dictated by the diameter of the sea floor tieback connector, which is deployed through the stem. These connectors can be up to 50xe2x80x3 in diameter.
Solutions to this loss of buoyancy include:
1) adding compressed air to the annulus between the riser and the stem wall after the riser is installed, and
2) making the buoyancy cans integral with the riser so they are deployed after the tieback connector is installed.
Adding air to the annulus is efficient use of the stem volume, but the amount of buoyancy can be so large that if a leak occurs there could be damage to a riser. The buoyancy tanks are usually subdivided so that leakage and flooding of any one, or even two, compartments will not cause damage.
Making the buoyancy cans integral with the risers has been used, but this requires a relatively small can diameter for deployment with the floating production platform, and the structural connections between the cans and the riser are difficult to design.
The circular geometry of the cans leaves areas of the centerwell between cans flooded.
The buoyancy cans are typically constructed out of steel and their weight can be a significant design issue. The first spar buoyancy cans were designed to withstand the full hydrostatic head of the sea, and their weight reflected the thicker walls necessary to meet this requirement. Subsequent designs were based on the cans being open to the sea at their lower end, with compressed air injected inside to evacuate the water. These cans only have to be designed for the hydrostatic pressure corresponding to the can length, and this is an internal pressure requirement rather than the more onerous external pressure requirement.
It has been recognized that it would be advantageous to develop a buoyancy system with greater structural capacity, lighter weight, and greater buoyancy.
The invention provides a buoyancy system that can be connected to a riser to provide buoyancy for the riser. The riser can extend substantially from a floating platform on or under the ocean""s surface, to the floor or the ocean. The buoyancy system includes a rigid ecto-skeleton couplable to the riser and defining an interior cavity configured to receive the riser therethrough. The ecto-skeleton can be movably disposed in the floating platform, and can withstand lateral and bending loads. A buoyant vessel is disposed in the interior cavity of the ecto-skeleton, and contains a buoyant material to provide buoyancy for the riser. The buoyant material can include air or pressurized air. Thus, the buoyant vessel can withstand pressure loads. When submerged, the buoyancy system, or ecto-skeleton and buoyant vessel, provides buoyancy for the riser, while withstanding lateral and bending loads.
In accordance with a more detailed aspect of the invention, the vessel can include a fiber composite vessel with a vessel wall including a fiber composite material.
In accordance with another more detailed aspect of the invention, the ecto-skeleton can include a plurality of members forming an external framework. The members can include 1) longitudinal members oriented longitudinally with respect to the framework, and 2) lateral members oriented laterally with respect to the framework, the longitudinal and lateral members being connected at intersections.
In accordance with another more detailed aspect of the invention, the members of the framework can include tubular members having hollow interiors with a buoyant material disposed therein. In one aspect, the ecto-skeleton has neutral buoyancy. Thus, the exto-skeleton itself contributes to buoyancy.
In accordance with another more detailed aspect of the invention, a plurality of cladding members can be disposed in gaps between proximal members. The cladding members can include a buoyant material to further contribute to buoyancy and efficiently utilize space in the floating platform.
In accordance with another more detailed aspect of the invention, the ecto-skeleton can have a square cross-sectional shape. The vessel, however, can have a circular cross-sectional shape. A plurality of inserts can be disposed in the ecto-skeleton between the framework and the vessel at corners of the square cross-sectional shape. The inserts can include a buoyant material to further contribute to buoyancy and efficiently utilize space in the floating platform, and in the ecto-skeleton.
In accordance with another more detailed aspect of the invention, the buoyancy system can be modular. Thus, the ecto-skeleton can be a first ecto-skeleton and include a second ecto-skeleton attachable to the first. A plurality of mating protrusions and indentations can be disposed on the first and second ecto-skeletons.
In accordance with another more detailed aspect of the invention, the ecto-skeleton and the vessel can have a circular cross-sectional shape.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.